As a conventional power semiconductor device having a trench gate structure, there are a MOSFET and an insulated gate bipolar transistor (hereinafter, referred to as IGBT). An electrical characteristic inspection is performed to a semiconductor device to determine whether the semiconductor device is a non-defective product having a desired characteristic. For example, in the case of the IGBT, a standard value serving as a criterion for determining whether the device has a desired electrical characteristic is set, and a Vg-Ic characteristic indicating a relation between a gate voltage Vg and a collector current Ic when the IGBT is turned on is measured. When the relation indicated by the Vg-Ic characteristic is within a standard value range, it is determined that the device is a non-defective product. When the relation is out of the standard value range, e.g., when the relation falls below the standard value, it is determined that the device is a defective product.
There may be a potentially-defective product, which has a characteristic within the standard value range at the stage of electrical characteristic inspection but would be deteriorated during usage of the product and would have a characteristic out of the standard value range. Such a defective product cannot be specified in the electrical characteristic inspection. In this regard, a new and useful method is desired.